Directional microphone systems are known. FIG. 1 is a block diagram illustrating a known method for implementing a directional microphone system 1. The system 1 includes a front microphone 2, a rear microphone 3, a delay 4, an adder 5, and an equalizer 6. The microphones 1, 2 are typically omnidirectional pressure microphones, but matched, directional microphones are also used. The system 1 forms a directional response pattern, with a beam pointing toward the front microphone 2, by subtracting a delayed rear microphone signal from a front microphone signal. The equalizer 6 then equalizes the directional response pattern to that of a single, omnidirectional microphone. In this manner, a variety of directional patterns can be implemented by varying the amount of delay.
Typical directional hearing instruments include a directional microphone system 1, such as the one illustrated in FIG. 1, having a two microphone first order differential beamformer in which a 6 dB per octave roll off in the low end of the frequency response is realized. As a result of this decreased signal strength at lower frequencies, typical directional hearing instruments have a reduced signal to noise ratio (SNR). Thus, the frequency response is typically equalized, as shown in FIG. 1, by applying gain at lower frequencies. Internally generated microphone noise, however, is typically amplified along with the signal, minimizing the improvement to the SNR of the microphone system 1. Similarly, wind noise is typically higher in directional hearing instruments due to the additional gain required to equalize the frequency response.
FIG. 2 is a graph 7 illustrating noise amplification (in dB) 8 in a typical directional microphone system 1, plotted as a function of frequency. The noise amplification 8 plotted in FIG. 2 is typical for a conventional, two microphone system, as shown in FIG. 1, with a port spacing of 10.7 mm and a hyper-cardioid beam pattern. As illustrated, the amount of noise amplification, i.e., the microphone self-noise, in a typical microphone system 1 increases at low frequencies and, at 100 Hz, the microphone self-noise may be amplified by 35 dB.